There's no problem with systems which allow interaction by email. It can be handy, and if you like that, then the Debian BTS is convenient for responding to bugs as a maintainer. However, email-
only systems which don't offer any other methods of interaction can be problematic.
email-only interaction can be massively inefficient. For larger changes, I might need to change the severities, tags, and follow up to dozens of bugs. For every metadata change I make, I need to check the reply to make sure the change took place. That means manually tying together sent emails with BTS replies and repeating any which failed.
I used to track all this on large pieces of paper! I shouldn't need a manual system to manage my interaction with an issue tracking system. When I press submit in any other system, the effect is immediate. This is hugely costly in terms of time and effort to do really trivial and mundane things. It's one thing to do it once, but what happens when you make 30 changes and have to wait 20 minutes for each request to round-trip? It's a logistical nightmare which can take several hours to complete.
If I was to highlight a single problem with the Debian BTS, it's that there was a failure to acknowledge its inefficiencies and look at other systems. The criticisms raised in this thread aren't new; they were known 15+ years back. There's a reason why the others are not using email as their primary mode of interaction, and it's a shame the BTS didn't get a decent web frontend to make it more efficient as well as more accessible.